This invention relates to gas burners, and particularly to inshot gas burners having a flame retention head which is radiant in operation thereby to permit operation at higher primary aeration and increasing the burning velocity resulting in low nitric oxide and nitrogen dioxide emissions, as well as low carbon monoxide emissions, and a quiet, stable compact flame.
Inshot-type burners used in gas furnaces and other gas fired appliances have an inlet into which combustible gas is fed under pressure and into which primary air or aeration is drawn, the burner having a venturi configuration, i.e., a reduced cross section between its ends, which produces a low pressure for drawing in the primary air. The air and gas are mixed in a diverging portion of the burner downstream of the venturi and burned as the mixture exits from the outlet end of the burner. Secondary air, i.e., additional air required to complete combustion, is induced into the burning mixture from outside the burner just prior to the flame entering the heat exchanger of the furnace or appliance. To enhance the flame stability to prevent the flame from burning downstream of the outlet end or head of the burner, a flame retention device in the form of an insert may be placed at the outlet end or head of the burner. The primary air-gas mixture flows through the insert and burns as it exits. If the flow velocity of the air-gas mixture is too great relative to the flame speed, i.e., the burning velocity, the flame will lift off the exit end of the burner body of the insert resulting in flame instability, noise and carbon monoxide. If the flow velocity of the air-gas mixture is too slow relative to the flame speed, the gas will burn within the burner itself, a condition known as flashback, resulting in overheating of the burner.
Flame retention in the prior art have been solid mass devices generally formed from sintered or powdered metal into small blocks. An example is the insert illustrated in U.S. Pat. No. 5,108,284 which describes a block having a large central opening with a plurality of teeth extending radially at the periphery of the block. A plurality of smaller apertures disposed radially about the central opening and also having a series of openings formed by stepped notches at the periphery is described in U.S. Pat. No. 5,186,620.
The aforesaid prior art flame retention inserts have generally only been concerned with flame stability, avoidance of excessive noise and limiting the production of carbon monoxide. This has been achieved by creating low velocity zones around the periphery of the outlet and/or eddy currents or recirculation vortex currents which help retain the flame while keeping primary aeration, i.e., the amount of primary air, at or below 50% of the total air required for combustion. The prior art burner heads or inserts are generally only stable to approximately 50% primary aeration. Above this primary aeration level, the burners become unstable and significant amounts of toxic carbon monoxide are produced, as is high noise. When burning fossil fuels such as methane the ideal products of combustion are CO.sub.2 and H.sub.2 O. In practice, when using air as the oxidant, the nitrogen in the air dissociates at high temperature and is oxidized to nitric oxide (NO) and nitrogen dioxide (NO.sub.2). The amount of NO produced is dependant on the maximum temperature and the time spent at this temperature, i.e., residence time. The formation of nitric oxide (NO) and nitrogen dioxide (NO.sub.2), both of which may hereinafter be referred to as NO.sub.x, has not been addressed by prior art burner designs.
Unfortunately, as the amount of carbon monoxide is reduced with the prior art burner designs, the amount of NO.sub.x is increased, and vice versa. The amount of NO.sub.x produced is dependent upon the maximum temperature and the residence time of the burning mixture at that temperature. The formation of NO.sub.x is undesirable since it contributes to acid rain and the formation of smog. For this reason California has imposed restrictions on the amount of NO.sub.x emissions of gas burning appliances. In the prior art, however, any reduction of the NO.sub.x emissions, required for the appliance, is normally achieved by the addition of a flame cooling insert in the first combustion tube of the appliance. This cooling insert has been made from a high temperature material, and thus is relatively expensive, and is also prone to failure, and may increase carbon monoxide levels.